Since a magnesium alloy can be used to reduce the weight of various products due to its low specific gravity, it is widely used in a package of a cellular phone and a portable sound equipment, a car component, a machine component, and a construction material. In order to achieve the more effect of low weight, it is necessary to make the magnesium alloy stronger and tougher. In order to improve such characteristics, the compositions of the magnesium alloy are to be optimized and the crystalline grain of the magnesium alloy is to be miniaturized. Especially, in order to miniaturize the crystalline grain of the magnesium alloy material, methods based on a plastic deformation process such as a rolling method, an extruding method and a drawing method have been used.
Japanese Unexamined Patent Publication No. 2005-256133 discloses a method for miniaturizing a crystalline grain diameter of a powder raw material by a roller compacter. More specifically, starting raw material powder is compressed and deformed through a pair of rolls and then formed into granular powder by a granulating process. The compression deformation and granulating process are performed repeatedly several tens of times, whereby the crystalline grain diameter of the powder becomes fine.
According to the method disclosed in the above document, since it is necessary to perform the compression deformation and the granulating process several tens of times repeatedly to obtain the powder having a fine crystalline grain diameter, there is room for improvement in view of production efficiency and economic efficiency.
Although the crystalline structure can be miniaturized by rolling a magnesium alloy plate material, basal sliding occurs at a low temperature (200° C. or less), since magnesium has a hexagonal close-packed lattice (HCP crystalline structure). Therefore, the degree of cold working of the magnesium alloy plate material is limited to several percents, and the rolling process is performed at 300° C. or higher in general. Even in this case, the rolling process must be performed at a rolling reduction of 25% or less in order to prevent the material from being cracked and fractured.
For example, “Structure and Texture of AZ31 Magnesium Alloy Plate Rolled at High Speed” in Abstracts of the 109th Autumn Conference of Japan Institute of Light Metals (2005) on pages 27 and 28 (by Tetsuo Sakai et al.) discloses a method for obtaining a fine crystalline structure by performing a high-speed rolling process for a magnesium alloy plate. Mr. Sakai et al. focused on the fact that it was necessary to increase a rolling reduction per passage to improve rolling efficiency and to use the rolling for the texture control, the fact that it was necessary to heat up the material to make high reduction rolling successful since only the basal sliding occurred in the magnesium alloy at a low temperature, and the fact that it was necessary to prevent a temperature being lowered due to heat transfer to the roller and a peripheral atmosphere during the process, in order to use heat generation in the process for the material maximally and increase the temperature of the material itself, and thus considered that it was effective to perform the process at high speed to decrease a contact time between the tool and the material, and tried the high-speed rolling. As a result, it has been found that since the rolling processability of the magnesium alloy is improved due to the high-speed rolling, the high reduction rolling can be implemented through one passage, so that an expanded plate material can have a fine grain structure and superior mechanical performance.
According to an experiment result provided by Mr. Sakai et al., it is reported that by the high-speed rolling at rolling speed of 2000 m/min, a rolling reduction of 61% can be implemented through one passage even at 200° C. as well as at 350° C. It is also reported that although a shear band is generated at a rolling temperature of 100° C. or less, as the rolling reduction is increased, a fine recrystallization grain appears in the shear band and the recrystallization grains are spread in the whole plate when the rolling reduction is high.
Although Mr. Sakai et al. estimate that a limit rolling reduction per passage is increased with increase of the rolling speed, a maximum rolling reduction confirmed in the experiment is 62% and it is not clear whether a rolling reduction higher than that can be implemented or not. In addition, according to the method by Mr. Sakai et al., the crystalline grain is miniaturized by use of dynamic recrystallization formed in the magnesium alloy plate during the high-speed rolling. When an extrusion billet is produced from the magnesium alloy material having the fine crystalline structure obtained as described above, and the extruding process is performed at a predetermined temperature, since the fine crystalline grain becomes large at the time of the extruding process, a crystalline structure of a magnesium alloy extrusion material obtained finally becomes large.